In a diver's open-circuit breathing apparatus, a regulator is used to provide breathing gas during a diver's inhalations. Typically, the regulator incorporates a mouthpiece coupled to a valve that opens (to provide breathing gas to the mouthpiece) under the control of a flexible membrane or diaphragm mounted in an exterior wall of the regulator. More specifically and as is well known in the art, the diaphragm/valve assembly is constructed such that a pressure balance is achieved across the assembly when a diver is under water. As a result of the pressure balance, the diaphragm flexes to cause the valve to open as the diver inhales.
This type of regulator works well when the diver's mouth and lungs are at the same height, i.e., when the diver is swimming horizontally or substantially perpendicular to the force of gravity. However, when a diver stands or swims with his head up, the lungs are deeper than the mouth and the water depth of the lungs is greater than the depth of the mouth. This generates a pressure difference between the outside of the chest and the gas inside the lungs that makes it harder for a diver to breathe than when there is no pressure difference. This situation is known as “static lung load”, “hydrostatic imbalance”, or “positive/negative pressure breathing”. For the diver who is standing/swimming “head up”, a mouth-held regulator imposes a negative hydrostatic imbalance. Conversely, if the diver is “head down”, a positive hydrostatic imbalance exists. It is well known that excessive imbalances will limit the diver's exercise capability and endurance.